travis: enable bundler caching in builds

From https://docs.travis-ci.com/user/caching:

On Ruby and Objective-C projects, installing dependencies via Bundler
can make up a large portion of the build duration. Caching the bundle
between builds drastically reduces the time a build takes to run.

.travis.yml

@@ -5,6 +5,7 @@ branches:
   - gh-pages
 sudo: false
 language: ruby
+cache: bundler
 script: "bundle exec rake validate && bundle exec rake lint && bundle exec rake spec SPEC_OPTS='--format documentation'"
 bundler_args: --without rake
 matrix: